My U.S. Pat. No. 4,790,143 discloses methods and apparatus for monitoring and protecting both a transport refrigeration system and the associated load in the load space to be conditioned by the refrigeration system. The monitoring method and apparatus detects the temperature of the air discharged into the load space by the refrigeration system, and the temperature of the air returning to the refrigeration system from the load space, and develops an algebraic difference signal. The sign of the algebraic difference signal is used to detect improper conditioning modes. When the conditioning mode is found to be correct, the absolute value of the difference signal is used in comparisons with predetermined reference values.
The detection of an incorrect mode, as well as a comparison which determines that the difference signal does not exceed the selected reference value, initiate a first timing period. The first timing period, if not reset by a subsequent detection or comparison which indicates a return to acceptable performance, will time out and issue a warning signal to the operator of the transport refrigeration system.
The appearance of the warning signal also reduces the magnitude of the reference value which is compared with the difference signal. If, when the warning signal is issued, the actual conditioning mode is not the same as the commanded mode, a second timing period is immediately initiated. Expiration of the second timing period before a return to consistency results in a shut-down signal being generated. If the actual and commanded conditioning modes are consistent, then the second timing period is initiated when a comparison between the difference signal and the smaller reference value finds that the difference signal does not exceed the smaller reference value. If the difference signal does not increase to a value which exceeds the reference value before the second timing period expires, a shut-down signal is provided which shuts down the transport refrigeration system.
Initiation of a defrost cycle resets both timing periods so that the sum of the two timing periods may be used to detect an extended defrost cycle.
The monitoring apparatus and methods disclosed in the hereinbefore mentioned U.S. Pat. No. 4,790,143 adequately protect both the transport refrigeration system and the associated conditioned load. However, when the monitoring apparatus detects a condition that merits shutdown of the refrigeration system, the operator does not know which of several conditions caused the shutdown. Thus, it would be desirable, and it is an object of the present invention, to provide a diagnostic function which will aid the operator and/or maintenance personnel in finding and correcting the cause of the shutdown.